GUIDE TO DISSECTION OF THE EAR
 

The dissection of the ear will be essentially a "do-it-yourself" project. This guide will outline the basic schema of the internal and middle ear for you, but you must decide on your own approach to the dissection. As part of your presentation you might explain what tools and what approach you took to achieve your dissection.

Note that your dissection should be carried out on one side only. Temporal bones from the other side will be harvested for later use by ENT residents.
 

SCHEMATIC OVERVIEW:

The ear and auditory apparatus can be divided into 3 portions:

1. The external ear: This consists of the auricle and external auditory meatus (which has both bony and cartilaginous components), which leads to the eardrum, or tympanic membrane.

2. The middle ear or tympanic cavity: The middle ear and its contents are the main object of today's dissection. Understand that the tympanic cavity is essentially a hollowed out cave in the petrous part of the temporal bone. The entrance to this "cave" is closed off by the tympanic membrane. The most obvious feature of the middle ear cavity is the auditory ossicles: malleus, incus, and stages (so named for their resemblance to a hammer, anvil and stirrup). The malleus attaches to the tympanic membrane at the "umbo" and is moved when the tympanic membrane is vibrated by sound waves. This in turn causes the incus to move; the incus transmits the vibrations to the stages. The footplate of the stapes makes contact with fluid in the inner ear by way of an opening in the bone called the fenestra vestibuli or oval window. The fluid in the inner ear is vibrated by the movement of the footplate of the stapes, transmitting the sound waves to auditory receptors in the inner ear. (Realize that since fluid is incompressible, there must also be another membrane-covered outlet for the inner ear to allow fluid to bulge out when the stapes vibrates. This opening is called the fenestra cochleae or round window.) The job of the ossicles is to provide approximately a 20X amplification of the vibrations hitting the tympanic membrane.Also housed in the middle ear are 2 small muscles which pull on the auditory ossicles: the tensor tympani and the stapedius.

Tensor tympani: This muscle attaches to the upper part of the handle of the malleus and pulls the tympanic membrane inward when it vibrates, rendering the tympanic membrane more tense. Its nerve supply is by way of the mandibular branch of the trigeminal nerve (same as for tensor palati).

Stapedius: This muscle attaches to the neck of the stages. It damps down the vibration of the stapes when it contracts. This minimizes transmission of overly loud sounds to the inner ear. The nerve supply to this muscle is by way of a small branch of the facial nerve, which, as you will see, passes through a bony canal in the petrous temporal bone just posterior to the stapedius muscle.

The middle ear as we've described it thus far is essentially an air filled cavity in the petrous temporal bone, sealed off from the outside world by the tympanic membrane. Problem: the volume of air occupying the tympanic cavity increases or decreases with changes in atmospheric pressure. When you fly in an airplane at twenty thousand feet, there has to be a way of letting air out of the middle ear cavity as the atmospheric pressure decreases and the volume of air in the middle ear expands. The role of allowing air to enter or escape from the middle ear to equalize pressure between the tympanic cavity and the atmosphere is carried out by the auditoria tube. The auditory tube runs from the nasopharynx (above the soft palate about 1 cm. behind the middle concha, Atlas Figure 9-57) to the anterior portion of the tympanic cavity. The walls of the auditory tube are cartilaginous over the first 2/3 of their length, but the tube runs through bone over the last third of its length before entering the middle ear. This tube is lined with a mucous membrane which is continuous with the mucosa lining the nasopharynx. Therefore, infections causing inflammation of the mucosa in the nasopharynx have the potential to spread to the middle ear via the auditory tube.

The tympanic cavity also communicates posteriorly with another hollow air filled cavity, called the mastoid antrum. Like the tympanic cavity, this cavity is lined with a mucous membrane and may be as large as 1 cm. in diameter. The opening by which the mastoid antrum communicates with the middle ear cavity is called the aditus. In the adult the mastoid antrum also communicates with mastoid air cells.

Figure 48.3 The 2 meatuses, which have blind ends, and the line of the airway (tube, cavity, aditus, and antrum), which passes between them, viewed from above.
Figure 48.4. Tegmen tympani and passages it covers.

Recall that the 7th and 8th cranial nerves leave the cranial cavity together by way of the internal acoustic meatus. You will then realize that the 7th nerve (facial) will pass somewhere in the vicinity of the tympanic cavity. In fact, it will descend to the stylomastoid foremen, through which it exits, by passing through medial wall of the tympanic cavity in the vicinity of the aditus of the mastoid antrum.. As it passes through the facial canal the facial nerve gives off the chorda tympani branch which passes directly through the tympanic cavity. The chorda tympani contains fibers relaying taste sensation from the anterior 2/3 of the tongue, as well as preganglionic parasympathetic fibers destined to synapse in the submandibular ganglion. As you have already seen, the chorda tympani joins with the lingual nerve in the infratemporal fossa.

The last important point to emphasize in this overview of middle ear anatomy is the relationship between the tympanic cavil>, and the great vessels of the head. Posteriorly, the jugular bulb sits beneath the floor of the tympanic cavity. The internal carotid artery passes anterior to the tympanic cavity. This relationship is well illustrated in the schematic drawings on the previous page and in the drawing below.
The inner ear: This contains the actual sense organs for hearing and the sense of balance: the cochlea (hearing) the semicircular canals (dynamic equilibrium), and the saccule and utricle (static equilibrium?). The cochlea looks rather like a snail shell, while the semicircular canals describe 3 half circles, oriented roughly at right angles to one another (see schematic below).

REGIONAL ANATOMY

You can best become acquainted with the regional anatomy of the middle and inner ears by studying it on a skull before you attempt the dissection. You will need fine wire or pipe cleaners to pass through various canals. Dental floss threaders also work extremely well for this purpose. You may also find a flashlight or a high intensity lamp useful for transilluminating the bones surrounding the tympanic cavity.

Start bit holding your skull so it is facing to your right, as in Atlas Figure 7-147. Identify the external auditors meatus. as well as the mastoid and styloid processes of the temporal bone.

Look into the external acoustic meatus. The tympanic membrane and auditory ossicles will be gone on these dried skulls. You will therefore be able to see most of the middle ear cavity and its bone walls.

Pass a blunt probe through the external acoustic meatus and explore the superior part of the tympanic cavity, which rises up above the opening. This is the epitympanic recess. In the intact specimen this recess houses of the malleus and incus. The roof of the middle ear cavity is called the seamen tympani, and is relatively thin.

Move the tip of the blunt probe now to the floor of the tympanic cavity. This too is relatively thin, and rests on the jugular bulb. Verify this by turning to the underside of the skull and passing a pipe cleaner into the jugular foremen. Use Atlas Figures 9-47 and 9-48 for orientation. Pass another pipe cleaner into the carotid canal and note the relationship between the internal carotid artery and the middle ear cavity.

Return to the lateral view of the skull, with the skull facing to your right. On the medial wall of the tympanic cavity you can see parts of the bony inner ear. Some of these are shown but not labeled in Atlas Figure 7-147. Using Atlas Figures 7-147 and 7-148 as a reference, locate the fenestra vestibuli (oval window) and fenestra cochleae (round window) and the promontory' which is the basal turn of the cochlea.

Now push your probe anteriorly and try to locate the opening of the auditory tube. Using Atlas Figures 9-47 and 9-48 as a reference, locate the canal for the auditory tube on the undersurface of the skull. Pass a fine wire through it and verify that it opens into the anterior part of the tympanic cavity. The tensor tympani follows a parallel course to that of the auditory tube and emerges just above it into the anterior part of the tympanic cavity. Returning to the lateral view of the skull, you may be able to see a prominence on the medial wall of the tympanic cavity that the tensor tympani hooks around to get to the malleus. This is called the processus cochleariformis, and is labeled in Atlas Figure 7-147.

In the posterior part of the tympanic cavity you may be able to see a prominence called the pyramid. The stapedius muscle arises from this and passes to the neck of the stapes.

Pass the tip of the probe to the posterior and superior portion of the tympanic cavity. Here you should find the aditus, the opening leading to the mastoid antrum. If the aditus is large enough you may be able to peer in and see the prominences of the facial canal and the lateral semicircular canal. Again using Atlas Figures 9-47 and 9-48 as a reference, pass a fine wire into the stylomastoid foremen (located between the styloid and mastoid processes) along the course of the facial canal. Returning to the lateral view, attempt to verify the course of the facial nerve in the facial canal by transillumination.

Now look at the inside of the cranial cavity, with your skull oriented as in Atlas Figure 7-34. Using Atlas Figure 7-161 through 7-163 as a reference identify the internal acoustic meatus. If possible, pass a tine wire or floss threader into the canal for the facial nerve (see Atlas Figure 7-164) and attempt to follow it all the way to the stylomastoid foremen. By placing a probe in the tympanic cavity and tapping on its roof. or by transilluminating try to identify the location of the tympanic cavity beneath the floor of the middle cranial fossa (use atlas Figure 7-163 as a reference). Pass a pipe cleaner into the jugular foremen and foremen lacerum and again verify the relationship between the tympanic cavity and the internal jugular vein and internal carotid artery (Atlas Figure 7-145A).
 

ACTUAL DISSECTION

Do not attempt the actual dissection until you are thoroughly familiar with the regional anatomy on the dried skull. You may want to keep the skull with you for reference as you do the dissection. As stated above, it is up to you to decide what approach you want to take with your dissection. However, below are some hints on how you might proceed. You are not limited to these suggestions, and you may choose to use some combination of approaches.

Lateral approach: Use Atlas Figure 7-147 as a guide. Remove the auricle and cartilaginous portion of the external acoustic meatus. Open up the mastoid bone behind the meatus to gain access to the mastoid antrum and enlarge the dissection from there.

Dorsal and medial approach: Use Atlas Figure 7-145A as a guide. Start chipping away petrous temporal bone above the jugular foremen.

Dorsal approach: Use Atlas Figure 7-163 as a guide. Mark the expected location, of the tympanic cavity beneath the floor of the middle cranial fossa and start chipping away bone from there.

Suggested dissecting tools: The best tools to use for this dissection are rongeurs (please treat these carefully as we have only a few, and they are expensive- clean and dry before putting them away) or a hemostatic forceps (very useful for picking away thin plates of bone).